Spread spectrum communication systems spread the frequency spectrum of the transmitted signal by modulating the carrier with a spreading signal. This spreading signal is obtained by multiplying pseudonoise (PN) codes by the data being transmitted. The spectrum of the spreading signal approximates that of white noise, so the resulting transmitted signal has a wide bandwidth.
Many different PN codes may be used, and the correlation between these codes is usually limited. Therefore, a spread spectrum system is very secure against eavesdropping, has a high efficiency of frequency, and is very robust to noise. For these reasons, spread spectrum systems are expected to be the primary means of carrying mobile and personal communications and wireless LANs in the future.
For such applications, however, demand is growing for higher data capacities, more efficient use of bandwidth, and faster data transfer than conventional spread spectrum techniques can provide. The present invention extends spread spectrum technology to allow systems with higher rates of data transmission.
The outline of the transmitter of a spread spectrum (SS) communication system using quadrature phase-shift keying (QPSK) modulation is shown in FIG. 26. In the figure, 102 and 105 are binary phase-shift keying (BPSK) modulators, 107 is a PN code generator (PN.G) for generating PN code sequences, and 108 is a phase shifter for shifting the phase of a carrier wave by .pi./2.
Data generated by a data generating portion (DATA1) 100 are added to a PN code generated by PN.G 107 in an adder 101. Data generated by a data generating portion (DATA2) 103 are added to a PN code generated by the PN.G 107. In this case, one complete cycle of PN code is used to encode each bit of the data. The addition performed in adders 101 and 104 is modulo 2, that is, a PN code is outputted as is when the data bit is 0, and the PN code is inverted and outputted when the data bit is 1.
An output of an adder 101 is inputted to a BPSK modulator 102, and a carrier wave generated by a carrier wave oscillator 109 is modulated by BPSK. An output of an adder 104 is inputted to a BPSK modulator 105, and a carrier wave is modulated by BPSK, which wave is generated by a carrier wave oscillator 109 and is shifted in phase by .pi./2 by a phase shifter 108. An in-phase component of QPSK modulation (component I, hereinafter) can be obtained from a BPSK modulator 102, and an quadrature component of QPSK modulation (component Q, hereinafter) can be obtained from a BPSK modulator 105. These two components of BPSK modulation are added in an adder 106 to create a QPSK modulated signal. The QPSK modulated signal is transmitted from antenna 110. In this way, a multiplexed spread spectrum QPSK signal is transmitted from the transmitter portion.
The structure of the receiver is not shown. When a spread spectrum multisignal is received, it is divided into components I and Q, and both components of data are demodulated by correlation calculation of the received signal with the same PN code as was used on the transmission side. The data encoded by an inverted PN code causes a negative correlative output, and the data encoded by a noninverted PN code causes a positive correlative output.